1. Field of the Invention
The present invention relates to a ZigBee system (IEEE 802.15.4) that is a short-range wireless personal area network, and more particularly, to a symbol detector for detecting a symbol in a receive modem of a 2.4 GHz ZigBee system.
2. Description of the Related Art
Recently, the term “ubiquitous” has been proposed to represent a communication environment in which one can connect to a network at any time, any place. And, there have been active researches on small-scale wireless communication systems, for example, a wireless Personal Area Network (PAN), a sensor network, a Radio Frequency Identification (RFID) rather than on large-scale communication networks such as a cellular network.
Ubiquitous computing is based on the premise of using at any time, any place and absorption into objects and environment in the real world to be integrated into everyday life. In addition, ubiquitous network represents an information communication network in which anyone can use at any time, any place without being restricted by communication speed, and can freely distribute information and contents therethrough. Realization of the ubiquitous network allows the user to be free of various restrictions with existing information communication networks or services and freely use information communication services. There have been efforts to develop new services using ubiquitous computing and network, and thus importance of technologies related thereto has been emphasized. Also, in the future ubiquitous computing environment, it is expected that a wireless sensor network will be composed of more than thousands of node objects with voluntary sensing, low-power communication capabilities to provide various information services at any time, any place.
In an effort to prepare for the ubiquitous environment, there have been active researches and developments on key technologies for realizing ZigBee. Based on these technologies, IEEE 802.15.4 standard defines a physical layer and a link layer as follows.
In general, ZigBee refers to a low-rate IEEE802.15.4 Wireless Personal Area Network (WPAN). It refers to a network in which a frequency band is divided into three bands, and spreading and data rates are set differently for each band for communication, as shown in Table 1 below.
TABLE 1Data ParameterPhysicalSpreading ParameterBitlayerFrequencyChip RateRateSymbol Rate(MHz)Band (MHz)(Kchips/s)Modulation(Kb/s)(Ksymbol/s)Symbol868/915868-868.6300BPSK2020Binary902-928  600BPSK4040Binary24502400-2483.520000-QPSK25062.516-aryOrthogonal
ZigBee aims for small-size, low-power and low-price products. It has drawn attention as a technology for short-range of within 10 to 20 m communication market for wireless networking at home, office, etc. and for recently attention-drawing ubiquitous computing. As shown in Table 1, once an arbitrary physical layer is set for communication in ZigBee, spreading and data rates are determined accordingly for each frequency band, and these settings are applied to both transmission and reception sides for communication.
As ZigBee system aims for ultra-small, low cost, low power as suggested by IEEE802.15.4, a high-cost oscillator cannot be used at a reception end, and therefore, considering that a low-cost oscillator with low precision level is used, it is advised that the system is operable even at an error of ±80 ppm. Therefore, a non-coherent symbol detector is generally used to strengthen the system against frequency offsets.
FIG. 1 is a block diagram illustrating a non-coherent symbol detector in a ZigBee receive modem based on 2.4 GHz OQPSK modulation according to the prior art.
Referring to FIG. 1, the conventional symbol detector includes a multi-delay-differential filter 120, a plurality of multi-correlators 130, a plurality of adders 151 to 153, a maximum value selector 160 and a symbol demapper 170.
The above conventional symbol detector samples a received signal Re[r(t)] and Im[r(t)] received at a reception end by the predetermined number of sampling by A/D converters 111 and 112 to convert the signal to a digital signal Re[r(k)] and Im[r(k)] and inputs the digital signal to the multi-delay-differential filter 120.
The multi-delay-differential filter 120 delays the received signal by a plurality of predetermined delay times 1Tc, 2Tc and 3Tc, conjugates each of the delayed received signals, and multiplies each of the delayed signals by the received signal to output multi-delay-differentiated signals Dr,1Tc(k), Dr,2Tc(k) and Dr,3TC(k).
The outputs from the multi-delay-differential filter 120 are inputted into a multi-correlator 130. FIG. 1 illustrates only the multi-correlator 130 and the multi-delay-differentiated PN sequence 140 corresponding to symbol #0 out of symbols #0 to #15. But the symbol detector includes the multi-correlators and the multi-delay-differentiated PN sequences corresponding, respectively, to the rest of the symbols #1 to #15. That is, the symbol detector includes a plurality of multi-correlators 130 and multi-delay-differentiated PN sequences 140 provided in the number corresponding to that of symbols (0 to 15). A multi-delay-differentiated PN sequence 140 refers to a PN sequence that is multi-delay-differentiated through the same process as conducted by the multi-delay-differential filter 120.
The multi-correlator 130 complex-conjugates each of the multi-delay-differentiated signals Dr,1Tc(k) Dr,2Tc(k) and Dr,3Tc(k) outputted from the multi-delay-differential filter 120 with the multi-delay-differentiated PN sequences Ds,1TC (k), Ds,2Tc (k) and Ds,3Tc (k) corresponding to the particular symbol, using a plurality of multipliers 131 and adders 132. Then, the multi-correlator 130 integrates for one symbol period with an integrator 133. For the value integrated for one symbol period, a real part and an imaginary part thereof are respectively squared by a square calculator 134 to eliminate frequency offsets.
Each of the plurality of summers 151 to 153 sums the output values of 1Tc, 2Tc and 3Tc delay-differentiated signal correlators of the multi-correlator 130 provided for each symbol, thereby obtaining the magnitude of the energy of the received signal.
A maximum value selector 160 receives all the output values from the multi-correlator 130, and selects the greatest value as the output value imax of the detected symbol. The symbol value selected as just described is demapped into bit data by a symbol demapper 170 to obtain desired information bit.
However, this conventional non-coherent symbol detector includes a squaring process by the square calculator 130 in the multi-correlator 130, degrading its capabilities due to resultant square loss, and also is hardly realized as hardware.